There is a great variety of optical systems used for inspection/measurements of articles, reading/recording information in an information carrier, etc. These systems typically comprise an illumination means equipped with suitable light directing optics, and a detection means equipped with light collecting optics. A system of this kind always utilizes a focus error correction technique to ensure that the article under inspection (or an addressed layer of the information carrier) is located in a focal plane defined by the light directing optics.
One conventional auto-focusing technique is disclosed, for example, in U.S. Pat. Nos. 4,123,652 and 5,563,702, where the technique is used for reading a radiation-reflecting record carrier and for photomask inspection, respectively. According to this technique, a light beam scattered from the article is directed through anamorphic elements, and the shape of the light beam cross-section after passing these elements is monitored. The basic principle underlying the implementation is that a cylindrical lens produces astigmatism. A focused beam is first displaced from an in-focus point in a first direction and then in a second direction perpendicular to the first direction. The beam cross section therefore changes from being oblong in the first direction through circular to oblong in the second direction. Thus, an auto-focus beam is typically split into a pair of sub-beams, which propagate along mutually perpendicular optical paths onto a pair of detectors through a pair of astigmatic elements, respectively. The detectors are quadrature photodiodes. When the article is in focus, both detectors are illuminated by a circular beam. As the article goes out of focus, the horizontal diameter of the beam decreases on one detector and increases on the other. This changes the electrical output from the quadrature detectors and the focus correction signal is thus calculated accordingly.
However, the above technique detects focal point displacement by monitoring different points' locations. This is not suitable when dealing with a patterned article, because the pattern may produce intensity differences at different locations relative to a specific feature of the pattern. As a result, focus corrections may be applied to the detected, supposedly out-of-focus point, while being (i.e. inspecting) in an out-of-focus plane. In other words, the above technique cannot be used when illuminating a line, which is typically the case in the inspection of patterned articles.
Another technique, disclosed in Japanese Patent No. 01-202708 assigned to Nikon Corporation, JP, utilizes a pupil-dividing prism. The prism is placed in a pupil location, and forms an image of an illuminated area on a detector. The obtained image is divided into two spatially separated image parts, when the illuminated area is out of focus of a focusing optics. However, the same two-part image is provided on the detector in both before-focus and after-focus locations of the illuminated area. Consequently, this technique, even through it is capable of detecting the out of focus position, fails to provide correct information as to whether this out of focus position corresponds to the before-focus or after-focus location of the illuminated area, for generating a focus error correction signal.